The present invention relates to processes for the preparation of polyetherimides and, more particularly, to processes which reduce the formation of the undesirable byproduct phthalic anhydride m-phenylenediamine imide (hereinafter "PAMI") during the preparation of polyetherimides.
Because of their light weight, durability and strength, engineering thermoplastics are widely used. One class of engineering themoplastics are the polyetherimides to which the present invention is directed.
Polyetherimides are sold by the General Electric Company under the trademark Ultem.RTM.. Such products of the General Electric Company include Ultem.RTM.1000 and ULTEM.RTM.1010 which are polyetherimide resins derived from bisphenol A dianhydride and m-phenylenediamine with phthalic anhydride used as an end-capping and chain-stopping agent. Polyetherimide resins are well known in the art and are of considerable commercial value for use in molding compositions because of their excellent physical, chemical and thermal properties. The high glass transition and heat deflection temperatures exhibited by these polymers permit their use in high performance applications. The Ultem.RTM.1000 and 1010 products mentioned above, for example, find applications in the automotive, aerospace and electrical industries.
A number of processes for making polyetherimides have been disclosed. Generally, these polymers are prepared by reacting an organic diamine with an aromatic bis(ether dicarbonyl), i.e., an aromatic bis(etheranhydride) or an aromatic bis(ether dicarboxylic acid). Two processes which have been of particular interest are the so-called melt polymerization and solution polymerization processes. The basic melt polymerization process was described by T. Takekoshi and J. Kochanowski in U.S. Pat. No. 3,803,805. This process involves combining an aromatic bis(ether anhydride) and an organic diamine and heating the mixture under an inert atmosphere to form a homogeneous melt. Water formed during the polymerization reaction is removed at a temperature of up to 350.degree. C. In a preferred embodiment of the process, the final stage of the reaction is conducted under reduced pressure to facilitate removal of water. The basic polyetherimide polymerization technique has been improved by employing catalysts to enhance yields or reaction rates (for example, see Takekoshi, et al. U.S. Pat. No. 3,833,544 and F. Williams III, et al., U.S. Pat. No. 3,998,840, and Takekoshi, U.S. Pat. No. 4,324,882). In addition, the melt polymerization method has been adapted to the continuous mode by conducting the reaction in extrusion apparatus (for example, see Takekoshi, et al. U.S. Pat. No. 4,011,198 and Banucci, et al. U.S. Pat. No. 4,073,773).
Solution polymerization is generally conducted by reacting an aromatic bis(ether anhydride) and an organic diamine in an inert solvent at temperatures up to about 200.degree. C. With this procedure, water of reaction is typically removed by azeotropic distillation. The resulting polymer is generally recovered by mixing the reaction solution with a precipitant, such as methanol.
The reaction solvents employed for solution polymerization reactions are selected for their solvent properties and their compatibility with the reactants and products. High-boiling nonpolar organic solvents are preferred. (E.g., see Takekoshi, et al., U.S. Pat. No. 3,991,004). Dipolar, aprotic solvents and phenolic solvents can also be used, particularly when an aromatic bis(ether dicarboxylic acid) is used as the starting material (e.g., see Takekoshi, et al., U.S. Pat. No. 3,905,942).
D. Heath and J. Wirth (U.S. Pat. No. 3,847,867) disclose a method for preparing polyetherimides which involves stirring a solution of an aromatic bis(ether anhydride) and an organic diamine in a dipolar, aprotic solvent under ambient conditions to produce a polyamide acid and casting the polyamide acid solution on a substrate to facilitate the removal of the organic solvent. The cast polyamide acid film can then be heated at temperatures of 150.degree. C. or higher. After the initial heating, the cast film can then be heated to temperatures of from 200.degree. C. to 300.degree. C. to convert the polyamide acid to the polyetherimide.
A process for making polyetherimides which is particularly preferred from the commercial standpoint is disclosed in U.S. Pat. No. 4,417,044 to Parekh. This disclosure is incorporated by reference herein Parekh discloses the reaction of an aromatic bis(ether anhydride) with an organic diamine and a "chain stopping agent" in an inert solvent mixture to form a prepolymer. The prepolymer generally contains a substantial amount of polyetherimide, but also typically contains partially reacted oligomers and polyamide acid intermediate compounds. The prepolymer is subsequently subjected to a second process step wherein the mixture is formed into a thin film under solvent-volatilizing conditions to effect substantially complete solvent and water removal. Further heating of the reaction product, preferably in a second thin film evaporator, substantially completes the polymerization to the desired polyetherimide. Polyetherimide removed from the second thin film evaporator can be continuously extruded, air cooled and pelletized to form a resin product suitable for injection molding and other applications such as sheet production.
The Parekh solution polymerization process has proven to be a highly efficient process for the production of polyetherimides. Unfortunately, the production of sheet materials via thin film extrusion of polyetherimide resins produced via the Parekh process described above has been accompanied by the frequent formation (termed "plateout" in the industry) of an intractable coating upon the nip rollers of the sheet-forming equipment. The coating appears as a powder and can interfere with the quality of the sheet materials, for example by causing visible imperfections in the sheets.
Accordingly it is an object of the present invention to provide an improved solution polymerization process for the production of polyethrimides which results in lower formation of plateout materials. It is another object of the invention to modify the Parekh solution polymerization process to reduce the formation of plateout materials while retaining the many advantages of that process.